smtp protocol error

[Leon]

Hi there,

I'm getting an error when sending emails but not news. However if I
delay the email and re-send it from the *draft* group, it will
*succeed*.

The error is:

,----[ Backtrace ]
| Debugger entered--Lisp error: (error "Sending failed; SMTP protocol error")
|   signal(error ("Sending failed; SMTP protocol error"))
|   error("Sending failed; SMTP protocol error")
|   smtpmail-send-it()
|   #[nil " ‡" [message-send-mail-function] 1]()
|   gnus-agent-send-mail()
|   message-send-mail(nil)
|   message-send-via-mail(nil)
|   message-send(nil)
|   message-send-and-exit(nil)
|   call-interactively(message-send-and-exit)
`----

,----[ trace of SMTP session ]
| 250 OK

| RCPT TO:<XXXXX.xXXX@XXXX.com>

| 550-Insecure access forbidden: turn on TLS+AUTH;

| 550 See http://www.cam.ac.uk/cs/email/securehermes.html

| QUIT

| 221 ppsw-0.csi.cam.ac.uk closing connection

| - Peer has closed the GNUTLS connection
`----

My smtp config is:

,----
| (setq message-send-mail-function 'smtpmail-send-it
|       smtpmail-smtp-server "smtp.xxx.ac.uk"
|       smtpmail-starttls-credentials '(("smtp.xxx.ac.uk" 587 nil nil))
|       smtpmail-smtp-service 587
|       smtpmail-debug-info t
|       smtpmail-debug-verb t)
`----

I'm using gnus 5.11 from emacs 20060731.

Thank you for your help.

-- 
Leon